The present invention is directed to a method of preparation of pyrochlore structure compounds. Most particularly, the present invention is directed to a method of preparing lead and bismuth ruthenate and iridate pyrochlores. These pyrochlores have many uses including use as oxygen electrodes in electrochemical devices.
A number of various types of electrochemical devices have been developed over the past few years for the production of electrical energy by electrochemical reaction and obversely for the consumption of electrical energy to effectuate electrochemical reactions. Many of these devices rely upon a reaction involving oxygen (or air) as part of the mechanism to accomplish the desired result. For example, such devices may contain oxygen electrodes which are oxygen reducing cathodes in which oxygen is catalytically electroreduced. Alternatively, such devices may contain oxygen electrodes which catalyze the evolution of oxygen from water. In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, metal-air batteries, fuel cells, electrolyzers, metal electrowinning devices, etc., are among the well-known electrochemical devices which may contain oxygen electrodes. Typically, such devices contain electrocatalyst materials at one or more of their electrodes and precious metals, such as platinum (on carbon support) and silver (on carbon and other supports), are frequently, employed as electrocatalysts.
In addition, various electrocatalytic alloys, compounds and compound mixtures have been developed for these electrochemical devices to achieve more desirable systems. For example, U.S. Pat. No. 3,536,533 (Kitamura) describes the use of an alloy of gold, silver, palladium and at least one of platinum, rhodium and ruthenium as a fuel cell electrode electrocatalyst, and U.S. Pat. No. 3,305,402 (Jones et al.) describes the use of a combination of platinum and ruthenium oxides as an electrocatalyst. However, both references describe these catalysts as fuel cell anode (or fuel oxidation) catalysts. O'Grady et al., Technical Report No. 37, "Ruthenium Oxide Catalysts For The Oxygen Electrode", Contract No., N0014-67-A-0404-0006 (AD-779-899) Office of Naval Research, May, 1974 (National Technical Information Service) describes the use of ruthenium oxide as an electrochemical catalyst for both the generation of oxygen and the reduction of oxygen. U.S. Pat. No. 3,405,010 (Kordesch et al.) teaches that spinel-type electrode catalysts have been found to produce better activation of the electrode and improved electrolyte repellency of the electrode by the inclusion of ruthenium.
The foregoing prior art describes various types of electrodes including those which utilize iridium and/or ruthenium-containing catalysts. However, none of these references teaches or renders obvious the method of preparation as claimed herein.
Heretofore, many pyrochlore compounds such as the pyrochlore compounds Pb.sub.2 Ru.sub.2 O.sub.7-y (lattice parameter of 10.253 A), Pb.sub.2 Ir.sub.2 O.sub.7-y (lattice parameter of 10.271 A), Bi.sub.2 Ir.sub.2 O.sub.7-y, Bi.sub.2 Rh.sub.2 O.sub.7-y Pb.sub.2 Rh.sub.2 O.sub.7-y, Pb.sub.2 Pt.sub.2 O.sub.7-y and Cd.sub.2 Re.sub.2 O.sub.7-y, commonly referred to as lead ruthenate, lead iridate, bismuth iridate, bismuth rhodate, lead rhodate, lead platinate and cadmium rhenate, respectively, and similar compounds, have been known. For example, Longo, Raccah and Goodenough, Mat. Res. Bull., Vol. 4, pp. 191-202 (1969), have described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y and their preparation at elevated temperatures which are in excess of 700.degree. C. Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) has also described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y (including the pyrochlore compound Pb.sub.2 Ru.sub.2 O.sub.6.5 having a lattice parameter of 10.271 A) and their preparation at 700.degree. C. and 3000 atmospheres of pressure. U.S. Pat. No. 3,682,840 (Van Loan) describes the preparation of lead ruthenate at temperatures of 800.degree. C. and higher. These references do not teach that lead-rich or bismuth-rich compounds made by the present invention exist or that any of the pyrochlore compounds described herein may be prepared at temperatures below about 600.degree. C. in the manner described and claimed herein.
U.S. Pat. Nos. 3,769,382 (Kuo et al.) and 3,951,672 (Langley et al.) both disclose the preparation of lead ruthenate and lead iridate using various techniques at temperatures of at least about 600.degree. C., and preferably at higher temperatures. However, these references fail to recognize that the lead pyrochores may be obtained at generally lower temperatures, as described herein.
Bouchard and Gillson, Mat. Res. Bull., Vol. 6, pp. 669-680 (1971) describe Bi.sub.2 Ru.sub.2 O.sub.7 and Bi.sub.2 Ir.sub.2 O.sub.7 preparation and properties, including the fact that these compounds have high conductivity and small Seebeck coefficients. However, there is no teaching that these compounds are useful electrocatalysts in electrochemical devices. Derwent's Basic Abstract Journal, Section E. Chemdoc, Week No. Y25, Abstract No. 320 (Aug. 17, 1977), Derwent Accession No. 44866Y/25 describes electrodes for electrolysis of alkaline and carbonate solutions which comprise nickel-plated steel strips coated with high conductivity layers containing Cd.sub.2 Re.sub.2 O.sub.7, Pb.sub.2 Re.sub.2 O.sub.7 or Ni.sub.2 Re.sub.2 O.sub.7. These compounds are prepared by impregnating perrhenic acid and a metal nitrate such as Cd nitrate onto a nickel strip and baking at 350.degree. C. However, these compounds are all rhenates rather than ruthenates or iridates and are not taught to be prepared by the method of the present invention. National Bureau of Standards, Washington, D.C., Institute for Mat. Research, Abstract of Rept. No. NBSIR-75-742 (1974) describes the use of mixed oxides as oxygen-reducing electrocatalysts in acid fuel cells, including the use of barium ruthenate. However, of all materials suggested for such electrocatalysts, none are of the pyrochlore-type structure compounds made by the method of the present invention.
Trehoux, Abraham and Thomas, Journal of Solid State Chemistry, Vol. 21, pp. 203-209 (1977) and C. R. Acad. Sc. Paris, t. 281 pp. 379-380 (1975) describe the solution preparation of a pyrochlore compound of the formula ##STR1## The synthesis is carried out by adding a bismuth nitrate solution to a solution of 17% potassium hydroxide containing an excess of potassium hypochlorite. The reaction is carried out in this medium for 2 hours in a reflux type of apparatus at a temperature slightly higher than 100.degree. C. The method of synthesis and the product prepared are different in many respects from the synthesis method and products herein. The compound prepared in the cited reference is not an oxide but rather an oxyhydroxide which has a significant amount of protons incorporated into the bulk structure. Proton nuclear magnetic resonance experiments on the materials of the present invention show that they are oxides which do not have significant amounts of protons incorporated into the structure. The pyrochlore synthesized by Trehoux et al. is not a ruthenium or iridium containing compound and, in fact, is believed not to be an electrically conductive pyrochlore. The potassium hydroxide solution used in the Trehoux reference serves not only as a reaction medium but also as a constituent in the reaction since potassium is incorporated into the A site of the pyrochlore. In the method of the present invention where an alkali solution is employed, it is solely a reaction medium with no measurable amount of alkali metal cations incorporated in the pyrochlore compound which results from the synthesis.
Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6, pp. 109 et seq. (especially at 109-113) (1971), and C. R. Acad. Sc. Paris, Vol. 271, Seire C pp. 1313-1316 (1970) report the solution preparation of pyrochlore compounds having the formula Pb.sub.2 Sn.sub.2 O.sub.6.xH.sub.2 O where 0&lt;x&lt;1. The conditions of preparation are strictly defined as follows: equimolar quantities of lead and tin are reacted from solution in the presence of the complexing agent nitrilo-triacetic acid (NITA) such that the concentration of [NITA]/[Pb.sup.2+ ]=2. The pH of the reaction medium is fixed at 11 and the reaction is carried out for several hours at 80.degree. C. The compound prepared by Morgenstern-Badarau et al. is a hydrated oxide whereas materials made by the method of the present invention are oxides. The pyrochlore prepared in this reference, while it does contain lead, is not a lead ruthenate or iridate pyrochlore material prepared by the method of the present invention. In fact, the pyrochlore prepared by Morgenstern-Badarau and Michel is believed not to be electrically conductive. Morgenstern-Badarau et al. also specifically state that their preparation method is one which gives a solid product containing Pb.sup.2+. The method of the present invention is such that the solid product may contain a mixture of Pb.sup.2+ and Pb.sup.4+. While the presence of a complexing agent is required in the synthesis described in the cited reference, no such complexing agent is required in the method of preparation of the present invention.
In summary, there exists a formidable body of prior art describing the existence of various pyrochlores, their potential uses including uses as dielectric materials, and describing various metals and metal oxides as electrocatalyst materials. Notwithstanding such prior art, there is no suggestion or teaching that lead or bismuth ruthenate and iridate pyrochlore compounds may be made by the method of the present invention.